1. Field of the Invention
The present invention relates to a data storing and reproducing device represented by a hard disk drive, and specifically to a position control of a head for reading or writing data in the data storing and reproducing device.
2. Background of the Invention
A hard disk drive is one of the most prevalent external storage of a computer. A magnetic disk as a storage medium of the hard disk drive, as is generally known, has a sector as a minimum storage unit, which is defined by further radially dividing a track obtained by dividing a surface of the disk, which is shaped like the annual rings of a tree. A line recording density is made higher near the outer circumference of the magnetic disk. In a zoned bit recording method, which is mainly used as a data storing method of the magnetic disk currently, all the tracks are divided into some zones, and each of the zones has a constant line recording density. Consequently, since a sector size is typically 512 bytes, the track closer to the outer circumference of the magnetic disk has a larger number of sectors.
The hard disk drive includes a magnetic head for reading data stored in the magnetic disk, and for writing data in the magnetic disk. The magnetic head is attached to an actuator mechanism the movement of which is controlled by a voice coil motor (VCM).
At the time of reading or writing data with the magnetic head, the actuator mechanism is driven to move and position the magnetic head to a predetermined track. Such moving and positioning operation is called seek. The magnetic head is allowed to seek to the predetermined track using servo information stored in the magnetic disk as a clue.
The track of the magnetic disk is sectioned into a plurality of sectors as described above. Each of the sectors includes a data area for storing data and a servo area for storing the servo information. In the servo area, track identification information and a burst pattern are stored as the servo information. The track identification information is information indicating a track address of each data track. The track identification information is read by the magnetic head to enable determination of the track position where the magnetic head is currently positioned. The burst pattern includes areas arranged at a constant interval in a radial direction of the disk, each area storing a signal, and is constituted of a plurality of signal storage area arrays, each having the signal storage areas of different phases. A displacement amount of the magnetic head from the data track can be determined based on the signal outputted from the burst pattern. In data reading/writing of the magnetic disk, position control is carried out in the following manner. While the magnetic disk is rotated, the position of the magnetic head is checked by use of the servo information. If the displacement occurs, the actuator mechanism, that is, the magnetic head, is driven to correct the displacement. In order to position the head at the center of the track, the head has been heretofore positioned to the predetermined track by monitoring a position error signal (PES) value obtained from the burst pattern.
However, abnormal servo information, a noise current, a defect in a servo logic circuit, a software bug, or the like sometimes makes the head movement control impossible. In such a case, the head is sometimes turned into a so-called runaway state. Then, for example, the magnetic head and the magnetic disk collide with each other to be damaged. As means for preventing such runaway of the head, there are mechanical mechanisms such as a crash stop and a latch. However, since a driving current of the VCM becomes larger as the hard disk drive comes to have higher performance, the speed of the actuator at the runaway state overpasses a limit of the mechanical mechanism. In other words, there is no effective means for preventing the runaway of the head as a mechanical mechanism.
Japanese Patent Laid-Open No. 8(1996)-77734 discloses a data storing and reproducing device capable of reducing a moving speed of the magnetic head in the case that a ungeneration period of a sector pulse exceeds an allowable range. The data storing and reproducing device is effective for the runaway of the magnetic head caused by ungeneration of the sector pulse, but there are various kinds of factors causing the runaway of the magnetic head in addition to the ungeneration of the sector pulse.